1. Field of the Invention
The present invention relates to a semiconductor device and a production method therefor, especially, a method for bonding a semiconductor element (hereinafter sometimes referred to as a “die-attach method”).
2. Description of Related Art
Various bonding methods for mounting semiconductor elements such as transistors, ICs, and LSIs have been known. In addition, various bonding methods that are suitable with, among semiconductor elements, light emitting semiconductor elements such as light emitting diodes (hereinafter sometimes referred to as “LEDs”) and laser diodes (hereinafter sometimes referred to as “LDs”) have been know as well.
Conventionally, die-attach methods for semiconductor elements are roughly classified into two categories, i.e., bonding methods that use epoxy resin adhesives (hereinafter referred to as “resin bonding”) and bonding methods that use eutectic metals having an eutectic point at a high temperature of 300° C. or greater (hereinafter referred to as “eutectic bonding”) (see, for example, Patent Documents 1 and 2). Such die-attach methods are selected according to the similarity between the thermal expansion behaviors of a lead frame material and a substrate material on which a semiconductor element is to be mounted, as well as the reliability, cost, and like factors. For example, resin bonding is used for light emitting diodes for use in liquid-crystal back lights of small portable devices and like devices whose cost is given priority, and eutectic bonding is generally used for light emitting diodes for lighting purposes that are required to last a long time and for laser diodes that are required to be highly reliable.
A resin for use in resin bonding is mostly a thermosetting resin such as an epoxy resin. A paste in which a powder of a conductive material such as silver is dispersed is also a type of resin for resin bonding. In resin bonding, a liquid epoxy resin is heated to 150 to 200° C. for curing. Resin bonding is convenient in that curing can be readily accomplished at low temperatures of 150 to 200° C. In particular, thermal degradation of the thermosetting resin and melting of the thermoplastic resin can be avoided in a general-purpose surface-mounted semiconductor device in which a lead frame is molded in advance.
However, the heat generation due to the recent increase of light energy attained by light emitting diodes, laser diodes and like devices as well as the recent increase in input electricity; causes the resin itself that is used in resin bonding to deteriorate with time, resulting in problems such as discoloration and deterioration of the bonding strength. The glass transition temperature, which is an indicator in terms of temperature of the modulus of elasticity of a resin, is, because curing is performed at a low temperature, lower than the solder mounting temperature applied when a semiconductor device is mounted as an electronic component, and thus separation resulting from deterioration of the resin strength caused by thermal shock during solder mounting is likely to occur. Moreover, resin bonding that uses only an epoxy resin and resin bonding that uses a silver paste both have a problem in that, since they have poor thermal conductivity and insufficient heat releasability, light emitting diodes and the like become unilluminable.
On the other hand, eutectic bonding that uses an alloy of gold and tin can solve the aforementioned problems of resin bonding.
However, eutectic bonding requires heating to 300° C. or greater when bonding, and is therefore not applicable to widely used resin packages of PPA (polyphthalamide) or the like since such packages cannot withstand high temperatures. In addition, even if silver plating, which has high light reflectivity, is provided over the surface of a wiring board or a lead frame on which a light emitting diode is to be mounted, light extraction effect cannot be enhanced since eutectic metals have poor reflectivity.
Patent Document 1: JP 2004-128330A
Patent Document 1: JP 2006-237141 A